Current wireless communication system requires transmission of a high-volume and high-quality multimedia data through limited frequency. As a method for transmitting high-volume data using limited frequency, a Multiple Input and Multiple Output (MIMO) system was introduced. The MIMO system forms a plurality of independent fading channels using multiple antennas at receiving and transmitting ends and transmits different signals through each of multiple transmission antennas, thereby significantly increasing a data transmission rate. Accordingly, the MIMO system can transmit a great deal of data without expansion of a frequency.
However, the MIMO system has a shortcoming that the MIMO system is weak to inter-symbol interference (ISI) and frequency selective fading. In order to overcome the shortcoming, an Orthogonal Frequency Division Multiplexing (OFDM) scheme was used. OFDM scheme is the most proper modulation scheme for transmitting data at a high speed. The OFDM scheme transmits one data sequence through a subcarrier having a low data transmission rate.
A channel environment for wireless communication has multiple paths due to obstacles such as a building. In a wireless channel environment having multi-paths, delay spread occurs due to the multiple paths. If delay spray time is longer than a symbol transmission interval, inter-symbol interference (ISI) is caused. In this case, frequency selective fading occurs in a frequency domain. In case of using a single carrier, an equalizer is used to remove the ISI. However, complexity of the equalizer increases as a data transmission rate increases.
The shortcomings of the MIMO system can be attenuated using an Orthogonal Frequency Division Multiplexing (OFDM) technology. In order to overcome the shortcomings of the MIMO system while maintaining the advantages of the MIMO system, an OFDM technology was applied to an MIMO system having N transmission antennas and M reception antennas. That is, an MIMO-OFDM system was introduced.
FIGS. 1A and 1B are block diagrams schematically illustrating an MIMO OFDM system. FIG. 1A is a block diagram of a transmitting part in the MIMO-OFDM system, and FIG. 1B is a block diagram of a receiving part in the MIMO-OFDM system.
Referring to FIG. 1A, the transmitting part includes a serial-to-parallel (S/P) converter 101, a plurality of encoders 102, a plurality of Quadrature Amplitude Modulation (QAM) mappers 103, a plurality of inverse fast Fourier transform (IFFT) units 104, a plurality of cyclic prefix (CP) adders 105, and digital-to-analog converter (DAC) and radio frequency (RF) units 106. The S/P converter 101 divides transmission data into a plurality of data sequences before encoding the transmission data. The encoders 102 encode the data sequences, respectively. After encoding, the QAM mappers 103 modulate the encoded data sequences based on a predetermined modulation scheme such as Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 16 QAM, and 64 QAM. The IFFT units 104 transform the modulated symbols into time domain signals, respectively. The CP adders 105 insert a CP code for a guard interval into the time domain signals. Then, the DAC & RF unit 106 converts the CP inserted digital signals to analog signals and convert the analog signals to RF signals. The RF signals are transmitted through an antenna.
Referring to FIG. 1B, the receiving part includes a plurality of analog-to-digital converter (ADC) and radio frequency (RF) units 107, a plurality of CP removers 108, a plurality of Fast Fourier Transform (FFT) units 109, an MIMO receiver, a plurality of decoders 111, and a P/S converter 112. The ADC & RF units 107 convert RF signals to analog signals and convert the analog signals to digital signals. The CP removers 108 remove CP codes which were inserted for a guard interval and transfer the CP code removed signals to the FFT units 109. The FFT units 109 perform FFT on the input parallel signals which are the CP removed signals. The MIMO receiver 110 estimates transmission data symbols which are generated by FFT. The MIMO receiver 110 calculates a log likelihood ratio (LLR) from the estimated symbols. The decoders 111 decode data sequences transferred from the MIMO receiver 110 and estimate the transmission data, respectively. The P/S converters 112 convert parallel data modulated by each decoder 111 into serial data.
The MIMO receiver 110 generally uses a decision feedback equalizer (DFE), zero forcing (ZF), minimum mean square error estimation (MMSE), and bell labs layered space-time (BLAST). As described above, the MIMO receiver has a problem of low performance although the MIMO receiver has a comparative simple structure compared to maximum likelihood detection (MLD).